Physical and functional definition of the Drosophila Notch locus by P element transformation.

Notch is a developmentally regulated locus which controls the differentiation of various Drosophila tissues, among them the embryonic nervous system. Molecular analysis has suggested that Notch is defined by an approximately 40-kb transcription unit which is spliced into a 10.2-kb mRNA composed of nine exonic regions and coding for a 2703-amino acid long transmembrane protein that shows homology to the mammalian epidermal growth factor. Here, we define the 5' end of the transcription unit and determine the sequences deleted in a Notch mutation involving the 5' nontranscribed region. Using a Notch cosmid vector we demonstrate by P element-mediated transformation that all sequences necessary for Notch function are confined in an approximately 40-kb long genomic region. cDNA sequences are used to construct a 15-kb "minigene" which lacks most, but not all, introns and its functionality is also tested by P element transformation. We show that, unlike the cosmid vector which is capable of rescuing completely all Notch mutations, only certain phenotypes can be rescued by the "minigene." The functional implications of our findings are discussed.

The molecular genetics of the Notch locus in Drosophila melanogaster.

The Notch locus of Drosophila melanogaster profoundly affects differentiation of the central nervous system in the early embryo. Previous molecular studies suggested that the locus spans 40 kb of DNA and encodes a 10.5-kb poly(A)+ RNA. The results of genetic, cytogenetic, and molecular studies of newly induced and preexisting Notch alleles are reported. Molecular analysis of 5' flanking mutations defines a distal limit for the locus, and transcriptional activity of Notch in relation to that of flanking transcription units provides evidence regarding the proximal limit. Examination of a set of mutable alleles implicates a foldback transposable element as the basis of chromosomal instability, and shows that insertion sequences within the locus do not necessarily result in a mutant phenotype. The results are discussed with regard to existing developmental and genetic analyses, and a molecular model is proposed that attempts to explain the pleiotropic action of Notch.

A dual laser flow sorter utilizing a CW pumped dye laser.

A dual laser flow sorter has been constructed from an existing single laser system by incorporating a dye laser as the second laser source. The argon ion laser emission was used both as a pump laser and as a source by beam splitting before entrance to the dye laser. The emissions of the dye laser and pump laser beams were recombined and focused with the same optical train used in the single laser system. The imaging on the stream of the flow sorter provided for spatial and thus temporal separation of the exciting beams and subsequent sample emissions.

Selection of viable cells with known DNA content.

Cells doubly stained with Hoechst 33342 (H-33342) for DNA content and fluorescein diacetate for viability can be selected on the basis of both criteria using a single UV laser flow sorter. The selection is made possible due to resonance energy transfer occurring between the H-33342 and fluorescein fluorophores. Both a static fluorescence microscope and a dual laser flow sorter were used to demonstrate that energy transfer occurs in the doubly stained cells and that the sensitized emission in conjunction with the DNA emission can be used to select populations of cells with known DNA content and viability. The results indicate that fluorescein liberated by cellular esterases is freely accessible to the nucleus.

Fluorescence polarization and pulse width analysis of chromosomes by a flow system.

Isolated Chinese hamster chromosomes have been analyzed using a multiparameter computer-controlled cell sorter to obtain information about unique properties of individual chromosomes. Parameters other than DNA content were sought that would further aid in distinguishing among chromosomes. The polarized emission of the DNA-specific bis-benzimidazole dye Hoechst 33342 was measured for each class of chromosomes identified by a distinct peak, i.e., differeing in DNA content. The emission anisotropy values for all chromosome classes was constant (emission anisotropy = 0.30), and the same value was obtained for purified DNA in solution. Pulse width was found to be a good parameter for resolving chromosomes as a function of total emission in the case of the smaller chromosomes and orientation (i.e., arm length) for large chromosomes. A simple theoretical model for predicting the pulse shapes generated by arbitrarily oriented, thin, rigid rods was developed and applied to the evaluation of the experimental data.